Field emission electron sources, often referred to as field emission materials or field emitters, can be used in a variety of electronic applications, e.g., vacuum electronic devices, flat panel computer and television displays, emission gate amplifiers, and klystrons. Field emitters of etched silicon or silicon microtips have been known (see Spindt et al., "Physical Properties of Thin Film Emission Cathodes", J. Appl. Phys., vol. 47, pp. 5248, 1976), but require expensive and elaborate fabrication techniques. Additionally, such field emission cathodes suffer from relatively short lifetimes due to erosion of the emission surfaces from positive ion bombardment.
Others have deposited diamond coatings on silicon surfaces to use the intrinsic electronic properties of diamond, i.e., its negative or low electron affinity. Negative electron affinity means that conduction electrons can easily escape from a diamond surface into vacuum. For example, diamond has been deposited by chemical vapor deposition (CVD) upon silicon substrates for formation of field emitters (see Geis et al., "Diamond Cold Cathode", IEEE Electron Device Letters, vol. 12, no. 8, pp. 456-459, 1991. However, these attempts have yielded low current densities, estimated from about 0.1 to 1 amperes per square centimeter (A/cm.sup.2), these current densities requiring a high voltage for initial electron emission and accordingly, high power consumption. Recently, amorphic diamond thin films have been deposited upon substrates such as chrome or silicon by laser ablation (see Kumar et al., SID 93 Digest, pp. 1009-1011, 1993) to form field emitters. These field emitters have achieved current densities exceeding those achieved by the earlier silicon microtips or etched silicon, and have achieved light emission from a phosphor bombarded by electrons from such a diamond coated field emitting surface. In one such coating of diamond by CVD upon a silicon or molybdenum substrate, it was found that graphite impurities or graphite particle-like inclusions present from the diamond deposition may have resulted in improved field emission (see Wang et at., Electronics Letters, vol. 27, no. 16, pp. 1459-1461 (1991)).
Further work involving diamond-coated field emitters has been performed by Jaskie and Kane (see U.S. Pat. Nos. 5,129,850; 5,138,237; 5,141,460; 5,256,888; and 5,258,685). They describe, e.g., forming field emission electron emitters by providing a selectively shaped conductive/semiconductive electrode having a major surface, implanting ions as nucleation sites onto at least a part of the major surface of the conductive/semiconductive electrode, and growing diamond crystallites at some of the nucleation sites, to produce an electron emitter including a coating of diamond disposed on at least a part of the major surface of the selectively shaped conductive/semiconductive electrode. These emitters are essentially a Spindt-type microtip or cathode overcoated with diamond film. Also, Dworsky et al. (U.S. Pat. No. 5,180,951) have described an electron emitter employing a polycrystalline diamond film upon a supporting substrate of, e.g., silicon, molybdenum, copper, tungsten, titanium and various carbides, with the surface of the diamond film including a plurality of 111 crystallographic planes of diamond or 100 crystallographic planes to provide a low or negative electron affinity. Dworsky et al. teach that the supporting substrate can be substantially planar thereby simplifying the fabrication of the electron emitter.
Despite the recent advances, further improvements in current densities and electron emission efficiency of field emitters are believed necessary to reduce power consumption requirements in most applications. Other improvements are needed in reproducibility of the emitters, in the lifetimes of the emitters and in reduced fabrication costs of the emitters.
In fabricating electronic devices, such as a flat panel display, field emitters have typically been formed as small flat plates, often referred to as cold cathodes. Several such small flat plates have then been pieced together in the fashion of tiles to provide the electron emission for a larger flat panel display. This leads to distinct lines or gaps in the emission pattern around the edges of the small flat plates or tiles. There are presently no techniques to fabricate a field emitter having greater than about a few square inches in surface area. Accordingly, the ability to readily and easily fabricate field emitters having a surface area of greater than a few square inches, e.g., a surface area the size of the ever larger display, e.g., television, screen sizes, is desirable.
Despite the level of industrial activity in the area of field emission of electrons, numerous problems and difficulties remain.
It is an object of the present invention to provide a field emitter material having high electron emission efficiency and low voltage requirements, i.e., low voltage switch-on requirements.
Another object of the present invention is to provide a field emitter material having a longer lifetime or longer period of operation in the face of positive ion erosion.
A further object of the present invention is to provide an easily fabricated field emitter.
Still another object of the present invention is to provide a field emitter material having ease of fabrication into large, e.g., up to a square foot and larger, emission surfaces.
A still further object of the present invention is to provide electronic devices employing the field emission emitter materials of this invention.
Yet another object of the present invention is to provide field emitter materials suitable for providing a variety of field emitter cathode geometries.
Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the drawings and detailed description of the invention which hereinafter follow.